McLaren ended Formula 1s pre-season schedule rooted to the bottom of the lap charts after concluding their winter schedule with another frustrating day.
Double world champion Fernando Alonso missed more than four hours of track time at Barcelona's Circuit de Catalunya on Friday following an engine failure inside the opening hour.
The 36-year-old was completing just his seventh lap when his car came to a standstill at turn seven with 55 minutes on the clock.
Alonso returned to the track after the team's latest reliability issue was resolved, and there was some late encouragement for the Spaniard when he set the third fastest time of the winter in the closing minutes.
But his early-morning stoppage marked the third day of this week's concluding four-day test in which McLaren have run into trouble.
Alonso missed nearly seven hours of running on Wednesday following an oil leak, while his team-mate Stoffel Vandoorne broke down three times the previous day. Over the course of two weeks of testing, the British team have enjoyed just two trouble-free days.
Lewis Hamilton's world championship-winning Mercedes team passed through the 1,000-lap barrier on Friday following another bulletproof pre-season campaign, but McLaren have completed only 599 laps, the fewest of any of the 10 teams.
The Woking marque, a winner of 20 combined drivers' and team titles, finished last but one in the constructors' championship in 2017 and have failed to win a race in six years.
They are hoping to return to the sharp end of the grid this season following their decision to divorce from Honda and switch to Renault power.
But the team's racing director, Eric Boullier, still expects them to be off the pace of Mercedes, Ferrari and Red Bull when the season gets under way in Melbourne in a fortnight's time.
,"We are trying to get McLaren back to where it should be, but like anything, you don't do it in one day," Boullier said. "There will be a few glitches but we haven't lost our ability to design fast cars.
"The fight at the front will still be between Mercedes, Red Bull and Ferrari. The pack has got closer and we believe we belong in the middle of that one."
Defending champion Hamilton signed off his winter schedule by completing nearly a century of laps on Friday morning. Hamilton, who managed 97 circuits, was 11th on the timesheets with Mercedes yet to show their true one-lap pace.
Valtteri Bottas took the team's lap tally for the day to 201 after taking over testing duties from Hamilton in the afternoon. The Finn finished sixth in the order.
Earlier, Ferrari's Kimi Raikkonen set the second fastest time of the winter after he moved to within four hundredths of a second of team-mate Sebastian Vettel's unofficial track lap from Thursday.
Hamilton begins the defence of his title at Melbourne's Albert Park on March 25.